1. Field of the Invention
This invention relates to an apparatus for dehydrating and consolidating a porous optical fiber preform and a method of the same.
2. Description of the Background Art
Vapor-phase Axial Deposition and Outside Vapor Deposition are known methods of making an optical fiber preform. In these methods, SiCl4 and GeCl4, and hydrogen and oxygen are supplied to a burner as source materials and as fuel respectively. SiO2 particles are generated by flame hydrolysis reaction and deposited on a starting rod. A porous optical fiber preform made by these methods is successively heated in a furnace in order to be dehydrated and consolidated.
It is necessary to keep the heating atmosphere in the furnace exceptionally clean to obtain a transparent glass of sufficient purity. To accomplish this, a porous optical fiber preform is put in a muffle tube and separated from an insulation material that contaminates the atmosphere. The muffle tube is made of carbon or quartz.
Carbon is suitable for a muffle tube because it has an ability to match a bigger optical fiber preform as well as to withstand a rapid temperature change and high temperature equal to or more than 1600° C. in which a porous optical fiber preform is consolidated into a transparent glass blank. (For example, see Japanese patent application laid open No. 6-345469)
FIG. 2 shows a sectional view of a dehydration and consolidation furnace with a conventional muffle tube made of carbon.
An optical fiber preform 1 is hung at the central part of chamber 8 by a supporting rod 2, which penetrates through opening 3a of muffle tube 3 and opening 7a of furnace body 7. Muffle tube 3 is made of high purity carbon and constructed of muffle pieces, which are multistage-piled to cover a large-scale optical fiber preform. Packing made of carbon is applied on joints 3b of muffle tube 3, and the joints are sealed. Heater 5 and insulation member 6 are arranged in space 9 between muffle tube 3 and furnace body 7.
In chamber 8, gas for dehydrating and consolidating an optical fiber preform 1 is supplied through gas inlet 11 and drained through gas outlet 12. A mixture of helium and chlorine is used as this gas for example, (See Japanese patent application laid open No. 6-127964). In addition, furnace body 7 is provided with gas inlet 15 and gas outlet 16 and an inert gas, e.g., argon, is supplied to space 9 so as to prevent oxidation of heater 5 and the insulation material.
In a dehydration and consolidation furnace constituted as above, a sealing means is used between supporting rod 2 and openings 3a, 7a. The sealing portion at opening 7a can be gas tight where temperature is comparatively low, but the sealing portion at opening 3a cannot be gas tight where temperature is high. In addition, it is difficult for joints 3b to be completely gas tight even if they are sealed with carbon packing. Furthermore, connection portions of a gas inlet and a gas outlet with the muffle tube are not completely gas tight.
Chlorine gas for optical fiber preform processing reacts with water discharged from optical fiber preform 1 by heating, and a corrosive gas, e.g., HCl, is generated. If pressure in chamber 8 is greater than the outside, this corrosive gas leaks to the outside of muffle tube 3 through joints 3b and the sealing portion at opening 3a which are not completely gas tight. The corrosive gas that has leaked out into furnace body room 9 from chamber 8 corrodes furnace body 7 made of stainless steel alloy. In addition, there is a concern that this harmful gas may flow out through gas outlet 16 which is not connected to a harmful element remover.
Such a gas flow from chamber 8 into furnace body room 9 can be prevented by maintaining the pressure inside furnace body 7 higher than the pressure within muffle tube 3. However, in this case polluted gas generated from insulation material 6 leaks from furnace body room 9 to chamber 8, and a high purity transparent glass blank cannot be produced.